Porosity Effects on Oxidation of Ultra-High-Temperature Ceramics

Ultra-high-temperature ceramics (UHTCs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hbox {UHTCs}}$$\end{document}) are the materials of choice in aerospace applications involving extreme environmental conditions. A major challenge concerning these materials' performance is oxidation. Understanding the underlying oxidation mechanisms is essential for predicting applicability in specific aerospace circumstances. Utilizing a combined experiment-modeling approach, this work analyzes the effects of one of the major factors, i.e., porosity, on oxidation of a typical UHTC, namely hafnium diboride (HfB2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hbox {HfB}}_{2}$$\end{document}). Systematic assessments of porosity characteristics in the micrographs depicting oxide layer emergence from parent boride result in the quantitative measures needed for modeling the oxidation process. The combination of experimental and modeling results indicates that the pore fraction and radii independently affect the oxidation outcome. The current study provides evidence for each of these effects, and provides a basis for the modification of the utilized model in conjunction with the experimental results. The inclusion of tortuosity for narrow pores results in reasonable agreement with experiments. Analysis of pores' roughness and orientation provides justification for this possible modification. Results for ZrB2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hbox {ZrB}}_{2}$$\end{document} and TiB2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hbox {TiB}}_{2}$$\end{document} supply comparisons with those for HfB2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\hbox {HfB}}_{2}$$\end{document}. The outcomes therefore present insight into the fundamentals of oxidation that can help with materials processing and selection in extreme-environment aerospace applications.